This invention simplifies the design of an audio frequency amplifier (AF amplifier), and also eliminates noise arising at the time of switching frequency bands or switching to or from other sources, e.g., an amplitude modulation (AM) detector in a set. Such noise often occurs due to a sudden change in the D.C. voltage component of input signals to the audio frequency (AF) amplifier.
Heretofore, conventional frequency modulation signal radio receivers (FM receivers) have needed AF amplifiers exhibiting a wide level response to the input signal from the frequency modulation detectors. Moreover, multi-band type conventional FM receivers or combination sets with an FM receiver section and other signal source sections, such as a tape recorder, have used decoupling capacitors connected before the AF amplifiers.
However, the decoupling capacitors are difficult to fabricate when using integrated circuits (IC's).